Feature Surge Protection Correct selection for ultimate protection
Selecting the right devices for final circuit protection can sometimes be confusing due to the wide variety of modern MCBs, RCCBs and RCBOs available in many different types and sizes. Here, Eaton’s David Pitt offers some guidance for electrical engineers
Left: leading
manufacturers are now offering com- pact RCBOs that save space in consumer units or distribution boards
AC or Type A device is specified, as they provide the same functionality and more. Type S RCCBs are selective devices intended for use where cir- cuits include more than one RCCB.
T
hree types of device are commonly used for final circuit protection in modern electrical installations - miniature circuit breakers (MCBs) for over current and short circuit protec- tion, residual current circuit breakers (RCCBs) for earth leakage protection, and residual current breakers with over- current (RCBOs) that combine the func- tions of an MCB and an RCCB in a single device.
Since the introduction of the 17th Edition of the IET Wiring Regulations, RCBOs are being used more widely, but when looking at the characteristics of protection devices it’s easier to start by considering MCBs and RCCBs separately. When selecting MCBs, it’s essential to remember that their primary func- tion is to protect cables downstream of the device. They are available in ver- sions with Type B, C or D tripping characteristics - the main difference being the ability to handle current surges without tripping. Type B devices trip between 3In
to 5In
and are suitable for domestic instal- lations and light commercial applica- tions where switching surges are small. Type C devices trip between 5In and 10In
and are often chosen for domestic, commercial and industrial applications where electric motors are in use. Type D devices, which trip between 10In
and 20In , are often used
in industrial applications where equipment that produces high inrush currents - such as an arc welding
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RCCBs are also available in various different types. Type AC RCCBs are sensitive to AC currents and are suit- able for most domestic and commer- cial applications. Type A RCCBs provide additional protection and are sensitive to AC currents and pulsating DC currents. Type B RCCBs are sensi- tive to AC, pulsating AC and steady DC leakage currents. This type should always be used in photovoltaic (PV) solar energy installations where the inverter type could allow leakage of DC current. Type B+ devices are similar to Type B, but respond to AC leakage currents over a wider frequency range. Additionally, Type B and Type B+ devices can be used wherever a Type
Below: the variety of modern MCBs, RCCBs and RCBOs available in many different types and sizes means that selecting the right devices for your final circuit protection can sometimes be confusing
RCBOs Now let’s turn to RCBOs. In principle, as these are functionally an MCB and a RCCB in the same device, it would be possible to produce versions with a huge range of combinations of their characteristics. In practice, manufac- turers limit themselves to the most commonly used combinations, such as a Type B or C MCB characteristic with a Type A or AC RCCB characteristic. Where other combinations are needed, it often means that an RCBO can’t be used and protection must be provided using a separate MCB and RCCB. However, some manufacturers offer field fittable residual current units that can be used to convert an MCB into an RCBO, making a wider range of combi- nations possible. Two recent product developments are worth mentioning. The first is that leading manufacturers are now offer- ing compact RCBOs that save space in consumer units or distribution boards and are easier to wire. They are also available with the same combinations of characteristics as standard RCBOs. The second development is the intro- duction of RCCBs utilising digital elec- tronic technology. These offer an extensive range of characteristics and are particularly suitable for use in ‘dif- ficult’ applications, such as the protec- tion of circuits supplying large numbers of fluorescent lamps with electronic ballasts.
Summary
Of course, this article has only looked at operating characteristics for protec- tion devices and provides guidance on what device to choose for specific applications. However, when choosing individual devices, there are many other factors that must be taken into account including the current rating for MCBs/RCBOs and the sensitivity for RCCBs/RCBOs. Comprehensive guidance can be found in the IET Wiring Regulations to which reference should be made when designing any type of electrical installation. Additionally, leading suppliers of pro- tection devices are always happy to provide advice and guidance.
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